The long-term culture of potential target cells for murine leukemia viruses has been accomplished. Cell lines from NIH Swiss and C57BL/6 bone marrow and spleen cells have been established. These cell lines have remained in culture for over 5 months. Morphological observation revealed that these cells are myeloid. Histochemical and chromosomal analyses of the cells are in progress. In newborn mice, these myeloid cells have not caused myeloid leukemia or solid tumors. Thus, such cell lines may be useful in the development of in vitro assays for leukemia virus transformation. Culture of other target cells such as T cells and B cells has also been undertaken. The long-term culture of target cells of murine leukemia viruses may provide new tools for examining virus-host interactions as well as provide new insights into cell differentiation and viral leukemogenesis.